The invention describes a method for improving the impregnability of wood. Various wood species, such as spruce, Douglas fir, fir, oak or acacia, are characterized by favourable properties, such as resistance to checking and splitting as well as low distortion and torsion, straight fibers and minimal branching. These wood species are not widely used because they are difficult to impregnate with chemicals. This is generally true of the heart wood of all wood species. Even if the most efficient impregnating methods are used in wood protection, a wood preservative frequently does not penetrate deeper than 2-3 mm, while the standards in wood protection for timber in European hazard class IV (Ground Contact Timbers) call for a depth of penetration of at least 5-10 mm and of 6-30 mm for poles. For example an uptake of at least 63 kg of creosote/m3 of wood is required for palisades and of 90 kg/m3 for poles (figure for U.K. is 115 Kg/m3). This limit cannot be reached with the above-mentioned refractory wood species without additional pretreatment
At present the method of pretreatment most frequently used in wood protection is the very costly incising method according to which 3 cm deep holes spaced 4-8 mm apart are cut into the base of the pole or throughout the sawn timber member in order to promote the penetration of the wood preservative.
The reason for the poor impregnability is the behaviour of the pits during wood drying. Pits are connective openings between the wood cells which permit water transport. The anatomy of the pits is such that the two cell walls on either side of the middle lamella form a dome-shaped cavity and each has an opening called a pore. At this point the middle lamella is thickened to form the so-called torus. When the wood is dried, this thickened portion attaches to the border of the pore and seals it irreversibly in refractory wood species. The chemical background of this process has not been ascertained yet.
At present the most common method of improving the impregnability of wood by wood preservatives is the wet storage of wood in ponds for a period of several months, which promotes the development of bacterial populations that open the pits. However, this process cannot be controlled and leads to irregular results.
A more modem version of this method was developed for the pretreatment of pulpwood in which a mixture of selected strains of bacteria is used and which is said to result in a reduction of blue stain and resin degradation as well as reduction of the cooking time (WO 9636765). A similar effect is achieved in pulping also by pretreating wood chips with the fungus Ophiostoma piliferum (Blanchette, Farrell and Burnes 1992, Tappi 75, 102-106).
In wood protection attempts have been made to restore the pit passage by the action of a pectinolytic enzyme preparation (German Patent DD 292 864 A5). The disadvantage of these processes is that they are designed to degrade only one particular component of the pits, such as pectin, however apparently several components, not yet determined, are responsible for the pit closure.
The subject of the invention is a biological process in which the impregnability of the wood by wood preservatives is improved by growing fungi on the wood. Preferably fungi of the genus Trichoderma, such as Trichoderma viride SIWT T70, Trichoderma aureoviride SIWT T1 or the strains Trichoderma harzianum LC1, LC2, Hypocrea piluliferum LC3, or Gliocladium roseum LC4 as well as weakly wood-degrading fungi, such as Phanerochaete chrysosporium LC5 or Dichomitus squalens LC6 are employed. In the latter case these fungi are eradicated after incubation.
The use of living wood-colonizing fungi has the advantage over the use of enzyme solutions in that enzyme solutions have to be applied by a separate impregnating process, which poses a problem in particular with refractory wood species, whereas fungi may actively grow into the wood with the help of their filamentous hyphae. Another advantage of fungi is that they excrete the necessary mixture of enzymes and other low molecular size agents in the vicinity of the pits, which is required for the opening of the pit passages, but the composition of which has not been determined yet.
The inventive feature of the present process is that for improving the impregnability of wood it is inoculated with selected strains of fungi of the genus Trichoderma or Gliocladium, or with strains selected from the group of weakly wood-degrading fungi which are particularly suited to improve the uptake of the impregnating agent by the wood. Use of selected strains of Trichoderma or Gliocladium cause no decrease in strength of the timber and additionally have the effect of biological wood protection, for example Trichoderma harzianum LC1, LC2, Hypocrea piluliferum LC3 (EP 0 615 409 B1). The use of weakly wood-degrading fungi only results in limited damage to the wood during the period of treatment (usually below 10% strength loss).
According to the invention the weakly wood-grading fungi are eradicated after pretreatment as soon as the desired impregnating aim is achieved to avoid extended damage to the wood.
The decontaminating and growth promoting measures prior to inoculation enable the applied strains of fungi to rapidly grow into the wood without any appreciable competition by other organisms and to perform their pit opening activity there. The process makes it possible to use also wood species which are difficult to impregnate without expensive and time consuming mechanical pretreatment processes, such as the incising method, in the same fields of application as easily impregnable wood species thereby significantly expanding the market for such species.